The advent, development, and increasing use of electrical machines coupled to Pulse Width Modulated (PWM) variable speed inverter drives utilizing rapid switching devices such as Insulated Gate Bi-Polar Transistors (IGBTs) has resulted in an increase in rolling element bearing failures in electrical machines. The reduced bearing life and increased rate of bearing failures is due to currents flowing from the rotating shaft to the machine frame through the bearings. Such currents flowing through the bearing manifest as damage due to Electrical Discharge Machining (EDM) between the bearing raceways and rolling elements thus causing physical mechanical damage of the bearing raceways and rolling elements which may ultimately cause premature failure of the bearing assembly.
Shaft induced electrical currents are inherent to electric motors and generators due to mechanical asymmetries in the electrical machine. Additionally, due to the very high rate of switching frequencies of PWM inverter drives, the Common Mode Voltage (CMV) is increased over non-PWM driven machines. This increased CMV causes a difference in electrical potential between the rotor shaft and the stator frame of the machine. When the CMV potential exceeds the impedance of the bearings, a current is induced in the shaft, and the current passes through the bearings seeking ground, potentially causing EDM damage to the bearings.
FIG. 1 shows the potential electrical current paths in inverter driven machines. These consist of: Stator to shaft current; Shaft to rotor coupling current; Rotor to shaft current; and Stator to frame current. Both the stator to shaft and stator to rotor coupling current can potentially cause damage to the bearings.
FIG. 2 illustrates a schematic, cross sectional view of a bearing in static operation in an inverter driven machine. During static or stationary conditions the lubricant (e.g. oil or grease) is displaced between the rolling element and raceway mating surfaces in contact and under load thus causing a low resistance for electrical currents to pass between the rolling elements and raceway surfaces.
FIG. 3 illustrates a schematic view of a bearing in normal operation in an inverter driven machine. During transient, defined as startup condition greater than ˜15% of rated rotational speed, and normal steady-state operating conditions, a lubricant film thickness is generated and maintained between the rolling element and raceway surfaces. This Elasto-Hydrodynamic Lubricant (EHL) film thickness causes the mating surfaces to separate from contact and “ride” on a film of lubricant.
Many methods have been employed to prevent shaft induced currents from causing EDM damage to the bearings. Such technologies include di-electric ceramic coatings such as aluminum oxide or silicon oxide on the bearing external surfaces or bearing housing, use of Faraday shields to prevent the charge build-up on shafts, electrically conductive bearing grease, and shaft-contacting ground shunts made of electrically conductive materials such as copper, brass, or carbon.
Di-electric ceramic bearing coatings have been utilized with some success but have several drawbacks. The coating is very hard and brittle and can fracture during installation or after installation under load. The resulting cracks can allow current to pass through the bearing. Di-electric ceramic bearing coatings also have a limited di-electric strength. At high voltage potential, current can pass through imperfections inherent within the coating structure, causing an arc that compromises the coating, passing through the coating and potentially causing EDM damage to the bearing.
Faraday shields have been employed with success but are very expensive to implement in most applications in industry.
Electrically conductive grease allows the current to continuously pass through the bearing surfaces but may contain elements such as copper, carbon, or phosphorus which can cause excessive wear of the bearing surfaces leading to premature failure of the bearing.
What is needed in the art is a grounding system that is located very near to the bearing rolling element to raceway interfaces (i.e. integrated within the bearing), that remains an effective shunt system for a prolonged period of operation, requires no maintenance or replacement, and is integrated in the bearing assembly for ease of installation